A receiver may receive a desired (primary) signal contaminated by one or more interfering signals and possibly by noise, for example, with a high-gain, usually directional, antenna. For example, a desired satellite signal and multiple interferer signals can be received by a gateway receiver for a satellite network. The multiple interferer signals may be caused by the receiver, for example, receiving terrestrial interference. The multiple interferer signals can be canceled from the desired signal by use of a primary antenna and multiple reference antennas (usually low-gain antennas).
In the conventional art, when multiple interfering sources (I) are present, then multiple reference antennas (Nr) where Nr>=I are used to provide an interference canceller. However, in the prior art, complicated matrix-based solutions are required when there are multiple interferers and multiple reference antennas. Although the prior art discloses use of a Least Mean Squared (LMS) process as an interference canceller, the prior art discloses using LMS only when there is a single interfering source, as opposed multiple interfering sources, are present.
FIG. 1 illustrates a single reference LMS interference canceller of the prior art.
An LMS interference canceller 100 includes a multi-tap Finite Impulse Response (FIR) filter h that uses complex coefficients, a subtractor 102, a multiplier 104 and a coefficient updater 106. A primary received signal y1 and a reference antenna input y2 are provided to the LMS interference canceller 100 to produce an output signal out. The primary received signal y1 includes a desired signal and interferers. The reference antenna input y2 may include interferers, for example, background noise, terrestrial noise, a signal from an undesired transmission, or the like. The FIR filter h is adaptively adjusted by the LMS canceller 100. The signal model of the received signals is given as:y1=α1P+β1I y2=α2P+β2I where y1 and y2 are signals from the primary and reference antennas, respectively; P and I are the primary and interfering signals, respectively, and αi and βi are some complex coefficients that capture the signal gains between the signal sources and the antennas. In general, there may also be some path delay difference between the signals received at the antennas (not shown), which delay is adjusted for by allowing for a multi-tap FIR filter h to cover the delay differential.
The LMS canceller 100 operates by adjusting the tap weights with the coefficient updater 106 (usually complex coefficients) of h to decorrelate the output signal out from the reference antenna input y2 to the filter h (here y2). The subtractor 102 subtracts the output (the filtered reference signal) of the FIR filter h from the primary reference signal y1 to generate the output signal out. Thus, in the prior art, the filter h is adjusted so that the output signal out is uncorrelated with the interference received over y2. For the LMS canceller 100 to work well, 1) the reference antenna input signal y2 contains a sample of an interferer (not shown) with a high signal-to-noise ratio, and 2) the reference antenna input y2 also contains very little of the primary received signal y1. The LMS canceller 100 is in the prior art of FIG. 1. The adaptation of the coefficients of the filter h may be described as:Hn+1=Hn+2μon*Xn  [1]where Hn is the vector of tap values of the filter h at time n, Xn is the vector of input values to the filter h at time n, on is the output (residual) signal, μ is an adaptation speed constant, and {*} denotes a complex conjugate.
The prior art only disclosed a matrix arithmetic formulation for suppressing interference from multiple sources or reference antennas. The present teachings are computationally simpler, and are easier to implement using inexpensive digital hardware, for example, a Field Programmable Gate Array (FPGA) or the like. Moreover, for multiple reference antennas, the prior art also disclosed cascading (serializing) output of single reference input LMS cancellers to successively attempt to cancel signals from each reference from the primary received signal y1, but this technique is not generally effective. In contrast to this serial cancellation, the present teachings jointly cancel the multiple reference inputs in parallel and are effective.